Inverse geometry volume computed tomography systems

ABSTRACT

The present invention pertains to an apparatus and method for inverse geometry volume computed tomography medical imaging of a human patient. A plurality of stationary x-ray sources for producing x-ray radiation are used. A rotating collimator located between the plurality of x-ray sources and the human patient is also used. A rotating detector can also be used.

RELATED U.S. APPLICATION

This application claims priority to the co-pending U.S. provisionalpatent application Ser. No. 61/315,780, entitled “Inverse GeometryVolume Computed Tomography Systems,” with filing date Mar. 19, 2010,which is hereby incorporated by reference in its entirety.

BACKGROUND

Modern computed tomography (CT) scanners have the goal of covering alarge volume of the patient in a single rotation at very fast rotationspeeds. This objective is driven by demands of cardiac CT to cover theentire organ in less than a heartbeat. Impressive results have beenachieved with the current generation of CT scanners. However, thedownside of this development is the increased dose to the patient, theincrease in scatter, and the degradation of image quality in the outerslices due to cone beam artifacts. In particular, the increased dose inmedical imaging has come under scrutiny, with several published studiesdocumenting the elevated risk of cancer resulting from the radiationinvolved in medical imaging.

The manufacturers of CT scanners are attempting to address dosageconcerns with new developments. One common avenue being pursued is theuse of photon counting detectors. Photon counting detectors haveintrinsically higher detective quantum efficiency (DQE) than integratingdetectors and have a bias towards lower photon energies. These qualitieslead to increased image contrast, resulting in lower dose whilemaintaining image quality. Even more contrast enhancement and dosereduction can be achieved with the use of energy-resolving detectors.However, photon counting and energy resolving detectors aresignificantly more expensive than integrating detectors. This increasedcost is particularly challenging for conventional CT scanners that relyon detectors with very large areas.

CT manufacturers are exploring a variety of other methods to reduce thisdose while maintaining image quality. However, these improvements areexpected to be minor compared to that which may be gained by analternative CT system concept, inverse-geometry CT (IGCT). ConventionalCT utilizes a single focal spot X-ray source and a large-area detector,whereas IGCT utilizes a large-area, multi-focal spot X-ray source and asmall-area detector. IGCT offers higher dose efficiency and fasteracquisition times than state-of-the-art conventional CT systems. Thus,IGCT has the potential to overcome disadvantages with conventional CTand significantly out-perform conventional CT scanners.

In conventional CT scanners, each projection is of the entire field ofview and is obtained with a single focal-spot X-ray source and a largedetector. By contrast, inverse geometry systems utilize a large-areascanned X-ray source and a field-of-view projection is composed of manynarrower projections each acquired with a different detector location.The detector in an IGCT system is quite small compared to that in aconventional CT system. Thus, it is economically feasible to implementadvanced yet more expensive detector technologies in IGCT. However, IGCTas currently realized in prototypes faces difficulties in implementationdue to a large source array to be rotated at high speeds and significantchallenges from high power and cooling requirements of the source.

What is needed is a CT imaging system capable of producing rapid highquality images. Furthermore, the CT imaging system should provide lowradiation imaging.

SUMMARY

The present invention pertains to an apparatus and method for computedtomography medical imaging of a human patient. A plurality of stationaryx-ray sources for producing x-ray radiation are used. A rotatingcollimator located between the plurality of x-ray sources and the humanpatient is used for projecting the x-ray radiation through the humanpatient. A x-ray detector is used for measuring amount of the x-rayradiation passing through the human patient and striking the detector. Amethod for computed tomography medical imaging of a human patient isalso described. X-ray radiation from a plurality of stationary x-raysources is produced and directed towards a collimator. The collimator isaround the human patient and the amount of x-ray radiation striking adetector is measured.

These and other objects and advantages of the various embodiments of thepresent invention will be recognized by those of ordinary skill in theart after reading the following detailed description of the embodimentsthat are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

FIG. 1 is a diagram showing an exemplary fixed-source computedtomography imaging system of one embodiment of the present inventionwith three vacuum envelopes.

FIG. 2 is a diagram showing an exemplary isolated collimator-detectorassembly of one embodiment of the present invention.

FIG. 3 is a diagram showing an exemplary fixed-source computedtomography imaging system of one embodiment of the present inventionwith nine vacuum envelopes.

FIG. 4 is a diagram showing an exemplary uniform illumination pattern.

FIG. 5 is a diagram showing an exemplary illumination pattern thatprovides an increased flux in the central region of the collimator.

FIG. 6 is a diagram showing an exemplary illumination pattern thatprovides an increased flux in the central region of the collimator.

FIG. 7 is a diagram showing an exemplary source ring with linear sourcesof one embodiment of the present invention.

FIG. 8 is a diagram showing an exemplary arrangement of sources of oneembodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims. Furthermore, in the following detaileddescription of embodiments of the present invention, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will be recognized by one of ordinaryskill in the art that the present invention may be practiced withoutthese specific details. In other instances, well-known methods,procedures, components, and circuits have not been described in detailas not to unnecessarily obscure aspects of the embodiments of thepresent invention.

FIG. 1 is a diagram showing an exemplary fixed-source computedtomography imaging system of one embodiment of the present invention.Imaging system 100 comprises a ring of X-ray sources 101, 102, and 103with an inner diameter of 1 m. The source ring can be made of threeX-ray sources 101, 102, and 103 making a three-gap system as shown. Forthe three-gap system, there can be three separate source arrays, eachcontaining three electron guns in a single vacuum envelope. Each ofthese source arrays can have a large-area tungsten transmission target.The source-spot locations can cover the full 360 degrees, except for asmall gap of a few centimeters between each of these arrays. The axialextent of the source array can be 16 cm. There can be a fixedpre-collimator between the source arrays and the spinning ring. Thispre-collimator defines the possible locations of the source-spots.

Within the ring of X-ray sources 101, 102, and 103 can be a rotatingdetector/collimator assembly. In one embodiment, only detector 110 andcollimator 120 rotate. Collimator 120 can consist of an array of holeswith each hole capable of illuminating the entire detector array. Thecenter of the detector array can be diametrically across from the centerof the collimator arc.

In operation, each row of the collimator 120 can have X-rays firingthrough its holes starting, for example, with the trailing hole andmoving sequentially to the leading hole. The collimator rows can fire insequence. A “super-view” can be obtained after all holes of allcollimator rows have “fired”. Other firing sequences are possible.

The detector elements can be read after a source-spot fires. The axialwidth of the detector array can also be 16 cm. By using the same axialwidth for both source and detector arrays there are no rays outside ofthe region of interest in the axial direction. Thus there is no unusedexposure such as occurs in cone-beam systems.

Imaging system 100 can have a large, 100 cm diameter, stationary ring ofscanning X-ray source-spots. Inside the source-ring can be a rotatingring containing detector 110 and collimator 120. This rotating ring, organtry, obtains power and outputs the detector signals through aslip-ring. FIG. 2 is a diagram showing an exemplary isolatedcollimator-detector assembly of one embodiment of the present invention.Collimator 220, which can be mounted opposite detector 110, can have ahole-pattern that focuses the X-rays onto detector 110. Detector 110 canbe 6 cm by 16 cm and collimator 220 can span an arc of about 120 degreesand can have a width of 16 cm. Each collimator holes can illuminate theentire detector. This system design allows for rotation speeds of atleast three rotations per second with image quality comparable to aconventional CT scanner.

Detector 110 can be a 6 cm by 16 cm detector. The detector ASIC can bemodified to allow parallel current-integration readout and dual-energyacquisition.

Collimator 220 can be designed to attenuate 120 keV photons. It canconsist of nearly 9,000 holes with a hole pitch of 2.3 mm. Each hole canbe tapered and angled to project X-rays onto a 5-cm by 10-cm detector ata distance of 150 cm. Collimator 220 can also have approximately 600holes projecting onto a 6-cm by 16-cm detector at a distance of 100 cm.Collimator 220 can be curved and have a larger area.

X-ray sources 101, 102, and 103 can be designed for continuous operationat 25 kW and at a tube voltage that can vary between 70 KVp and 120 kVp.The focal spot size can be 0.4 mm and the spot dwell time can be 1 μswith a duty cycle of 80%. The complete collimator can be scanned every15 ms. X-ray sources 101, 102, and 103 can include a thin-film tungstentarget layer deposited on a water-cooled 25-cm-diameter beryllium disc.The source power can be increased to 50 kW.

For a three-gap system, X-ray sources 101, 102, and 103 must cover asignificantly larger target area. A large vacuum envelope that housesthree guns in each source can be used. Each gun can illuminate a thirdof the target area. The use of three guns enables the entire target areato be illuminated. Different window material such as stainless steel andaluminum nitride can be used.

The projection data can be acquired as the collimator-detector assemblyrotates around the patient. Collimator 120 can be located between thesource array and the patient and source-spots are active only whenbehind collimator 120. Collimator 120 moves only a small angularincrement during the time the scan of every designated hole incollimator 120 is completed. A complete scan of collimator 120 isdescribed as a “superview”. The maximum travel of detector 110 during anacquisition of a superview is one detector width. Therefore, a completedataset can be obtained with as few as about 60 superviews.

High-weight, high-voltage, and high-power components of imaging system100 can be removed from the challenging environment of the rotatinggantry. Miniaturization of the high-voltage power supply is notrequired. High-power slip rings are not required. The X-ray source arraycan be cooled with hospital water, eliminating the conventionalgantry-mounted radiator and increased air-conditioning requirement.Faster rotation times and faster volume acquisitions are possible.Overall reliability can be increased by the removal of many components,especially X-ray sources, from the high-G-force environment of therotating gantry. A total source array area that is approximately threetimes larger than conventional systems can be required. However, theengineering necessary for this is greatly simplified compared to arotating source array. Also, the cost per area for sources issignificantly less than the cost per area of detectors. Thus, this canbe also economically feasible.

FIG. 3 is a diagram showing an exemplary fixed-source computedtomography imaging system of one embodiment of the present inventionwith nine vacuum envelopes. In this embodiment, the source ring is madefrom nine individual X-ray sources forming a nine-gap system. There arenine separate source arrays, each containing a single electron gun in asingle vacuum envelope. Each of the envelopes can have a 400 cm2 sourcearea.

FIG. 4 is a diagram showing an exemplary uniform illumination pattern.FIG. 5 is a diagram showing an exemplary illumination pattern thatprovides an increased flux in the central region of the collimator. FIG.6 is a diagram showing an exemplary illumination pattern that providesan increased flux in the central region of the collimator. FIG. 5 andFIG. 6 show patterns with increased intensity in the center. In practicethe uniform and non-uniform patterns could be interleaved to ensuresampling completeness. The use of different illumination patterns canprovide a two-dimensional adaptive filter. Several schemes for selectingthe distribution of illumination are possible. Importantly, theillumination for one superview can be based on the results of theprevious superview.

Iterative reconstruction methods can also be used. In particular,Maximum Likelihood Expectation Maximization (MLEM) is well suited fordatasets from unconventional geometries. The algorithm is less prone tounder-sampling artifacts and tends to reduce noise compared to standardalgorithms.

One of the most critical design issues is the ability to produce enoughphotons to provide the desired image quality. Imaging system 100acquires enough photons to produce an acceptable image. The detectorarray is 60 mm by 160 mm giving an area of about 96 cm². The duty cycle(the source-spot on time) utilized of imaging system 100 can be 80%. Thesource of imaging system 100 can have a power rating of 50 kW. Comparedto a 85 kW tube, this reduces the number of photons by a factor of 0.59.Imaging system 100 can have a slightly shorter focus-to-detectordistance giving it a factor of 1.17 advantage.

Imaging system 100 does not rely on the anti-scatter grids used inconventional CT systems to reduce scattered radiation in the projectionimages. As discussed earlier, imaging system 100 takes advantage of thesignificantly smaller detector compared to a conventional system.Scatter scales approximately with the detector size assuming a constantdistance between patient and detector. The smaller detector of imagingsystem 100 can be a significant advantage as the amount of scatterscales with the illuminated volume that, for a fixed object, isproportional to the detector area. The amount of scatter can be lessthan 10% for imaging system 100 while for a conventional system scatterexceeds 40% In a conventional system, scatter is managed with ananti-scatter grid, whereas in imaging system 100, an anti-scatter gridwill not be necessary. The efficiency is about 75%. Imaging system 100can have a significantly lower scatter fraction and can be operatedwithout an anti-scatter grid, giving a photon advantage of 1.33.

Detector 110 can be photon counting, having an intrinsic DQE advantageof 20%. Additionally, photon counting detectors have a bias towardslower energies giving another 20% advantage. Thus, fewer photons areneeded for the same image quality and can be counted as a (virtual) fluxincrease of a factor 1.44.

The transmission anode of imaging system 100 can provide 1.7 times asmany photons for the same current as the more traditional steep-anglereflection anode.

Because imaging system 100 can adjust the number of photons dependingupon the thickness of the object on a view-by-view, or evenbeam-by-beam, basis, a significant increase in maximum number of photonscan be obtained. An average increase of a factor of 4 can be achieved.

The following table summarizes the cumulative advantages anddisadvantages, and shows that the number of available photons iscomparable to that of a standard system.

IGCT/Standard Cumulative IGCT property relative to standard 0.15 0.15smaller detector area 0.80 0.12 lower duty cycle for IGCT 0.59 0.07 lesstube power 1.17 0.08 shorter source-detector distance 1.33 0.11operation without AS grid 1.44 0.16 photon-counting detector 1.70 0.27transmission anode 4.00 1.08 virtual bow-tie

The duty cycle can be increased to 100%. Imaging system 100 can usemultiple tubes that can be alternated thus filling in the off-time of asingle source. In addition, both iterative reconstruction and energyresolving detectors can improve performance. Overall, imaging system 100can increase the effective number of photons by more than a factor oftwo.

Some of the effects discussed previously convert directly into dosesavings to the patient. Imaging system 100 does not rely on theanti-scatter grids used in conventional CT systems. Anti-scatter gridsare positioned after the patient and also prevent a significantpercentage of the radiation from reaching the detector. Thus removingthe anti-scatter grid reduces the dose to the patient. The omission ofanti-scatter grids, and similarly, the removal of the dead-space betweendetector elements, leads to about a 25% improved dose efficiency.

The implementation of an adaptive filter can be used with inversegeometry CT and imaging system 100. The effective intensity of eachsource-spot-to-detector beam can be adjusted depending on the patientthickness, or attenuation, for that beam. This adaptive approach alsominimizes irradiation where no body parts are present. A dose saving onthe order of a factor of two can be achieved. Photon counting detectorsprovide an additional dose savings of a factor of 1.44.

The combined dose saving with imaging system 100 is almost a factor of4. Even further dose savings can be achieved with the use of an energyresolving detector and iterative reconstruction methods. Imaging system100 can be used only to scan the organ of interest and thereby furtherreduce the dose to the patient.

FIG. 7 is a diagram showing an exemplary source ring with linear sourcesof one embodiment of the present invention. Rather than using atwo-dimensional array of sources, the source ring uses lines of sources.These linear sources can be constructed using either transmissiontargets or reflection targets. An array of linear X-ray tubes isarranged in a ring. Detector-collimator assembly rotates inside thering.

FIG. 8 is a diagram showing an exemplary arrangement of sources of oneembodiment of the present invention. An array of linear x-ray tubes isarranged in a ring. Detector-collimator assembly rotates inside thering. This arrangement of source-spots can achieve complete sampling asthe gap between any two linear sources is covered by a third linearsource as shown in FIG. 8. Every plane intersecting the ring alsointersects a source trajectory. As an example, although the dashed linelies in the gap between tube 2 and 3, it intersects tube 4 of FIG. 8.Another advantage is that the tube target, whether transmission orreflection, can be at a steep angle with respect to the X-ray beam. Thisallows a line-focus electron beam to be used which, in turn, enables afour-fold increase in tube power. The area source approach hasadvantages with the heat loading of the target and that implementationof the virtual bowtie is easier.

Imaging system 100 can have numerous advantages compared to conventionalCT systems. Imaging system 100 can have lower dose and can be four-foldmore dose-efficient than conventional systems. Imaging system 100 canhave faster volume acquisition with scan times less than 300 msec.Imaging system 100 can perform whole-organ imaging with no tabletranslation and no cone-beam artifacts. Data can be reconstructed usingexisting algorithms. Thus, advantages include fast acquisition and thereduction of dose, artifacts, and cost. Image quality can be comparableto standard CT and also have a significant margin to exceed currentperformance. Complete datasets can be produced and a variety ofreconstruction algorithms can be used for efficient reconstruction.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and many modifications andvariations are possible in light of the above teaching. The embodimentswere chosen and described in order to best explain the principles of theinvention and its practical application, to thereby enable othersskilled in the art to best utilize the invention and various embodimentswith various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto and their equivalents.

What is claimed is:
 1. A computed tomography x-ray imaging system forimaging a human patient comprising: a plurality of stationary x-raysources for producing x-ray radiation; a rotating collimator locatedbetween said plurality of x-ray sources and said human patient forprojecting said x-ray radiation through said human patient; and an x-raydetector for measuring amount of said x-ray radiation passing throughsaid human patient and striking said detector.
 2. The computedtomography x-ray imaging system of claim 1 wherein said plurality ofstationary x-ray sources are arranged in a ring around said humanpatient.
 3. The computed tomography x-ray imaging system of claim 2wherein said plurality of stationary x-ray sources are scanned x-raysources.
 4. The computed tomography x-ray imaging system of claim 1wherein said x-ray detector rotates at a speed of at least threerotations per second.
 5. The computed tomography x-ray imaging system ofclaim 1 further comprising: three vacuum envelopes wherein each of saidvacuum envelopes comprising three of said plurality of x-ray sources. 6.The computed tomography x-ray imaging system of claim 1 wherein saidplurality of stationary x-ray sources is an array of linear x-ray tubes.7. The computed tomography x-ray imaging system of claim 1 wherein saidcollimator spans an arc in the range of 120 degrees and less.
 8. Thecomputed tomography x-ray imaging system of claim 1 further comprising:nine vacuum envelopes wherein each of said nine vacuum envelopescomprising one of said plurality of x-ray sources.
 9. The computedtomography x-ray imaging system of claim 1 wherein said collimator isscanned with said x-ray radiation every 15 milliseconds.
 10. Thecomputed tomography x-ray imaging system of claim 1 wherein saidcollimator spans an arc of about 120 degrees.
 11. The computedtomography x-ray imaging system of claim 1 wherein said x-ray detectoris capable of rotating around said patient.
 12. The computed tomographyx-ray imaging system of claim 1 wherein said x-ray detector has an axialwidth in the range of 16 cm and less.
 13. The computed tomography x-rayimaging system of claim 1 wherein said x-ray detector has a length inthe range of 16 cm and less.
 14. The computed tomography x-ray imagingsystem of claim 1 wherein said x-ray detector has a width in the rangeof 5 cm and less.
 15. A method for producing a computed tomography x-rayimage of a human patient comprising: producing x-ray radiation from aplurality of stationary x-ray sources; directing said x-ray radiationtowards a collimator; rotating said collimator around said humanpatient; and measuring amount of said x-ray radiation striking adetector.
 16. The method of claim 15 further comprising: rotating saiddetector around said human patient.
 17. The method of claim 15 furthercomprising: scanning an electron beam over a target to produce saidx-ray radiation.